This invention relates to a projection type image display apparatus such as a liquid crystal projector of the reflection type and an optical system for use with a liquid crystal projector of the reflection type and the like.
A projection type image display apparatus is conventionally known which includes an illumination apparatus, a light modulation element for modulating illuminated light in accordance with an image signal, a demultiplexing optical system for illuminating light emitted from the illumination apparatus upon the light modulation element, and a projection optical system for projecting the light from the light modulation element to form an image. One of projection type image display apparatus of the type described is disclosed, for example, in Japanese Patent Laid-Open No. 2000-105360 (hereinafter referred to as Patent Document 1).
In the projection type image display apparatus, a discharge lamp is usually used as the light source, and a transmission type liquid crystal element, a DMD (Digital Micromirror Device) and the like are used frequently as the image modulation element. Further, in recent years, also a projection type image display apparatus has been put into practical use which uses a reflection type liquid crystal element having a higher resolution as the light modulation apparatus.
In the projection type image display apparatus, a light source which emits white light is used, and the white light from the light source is demultiplexed into lights of three colors of red, green and blue using a dichroic mirror. The lights of the colors are illuminated on corresponding light modulation elements. The light modulation elements individually modulate the illumination lights in accordance with red, green and blue image signals. Then, the illumination lights modulated by the light modulation elements are multiplexed by a color multiplexing element such as a cross prism and then projected on a screen through a projection lens.
Where a reflection type liquid crystal element is used as the light modulation element, polarized light is used. In this instance, light emitted from the light source is converted into light polarized in one direction using a polarization conversion element and then demultiplexed into lights of three colors, which are introduced to corresponding reflection type liquid crystal elements. FIG. 22 schematically shows a device configuration of a reflection type liquid crystal element and associated elements of a conventional projection type image display apparatus.
Referring to FIG. 22, the conventional projection type image display apparatus 110 shown includes a polarizing beam splitter (PBS) 111, a reflection type liquid crystal element 112, and a linearly polarizing element 113.
Where the polarization conversion element described above is used, it is difficult to obtain a high P-S conversion characteristic over a wide incident angle over the overall visible region. Therefore, in the conventional projection type image display apparatus 110, light is passed through the linearly polarizing element 113 to obtain a flux of light having a higher polarization degree, and the flux of light thus obtained is introduced into the polarizing beam splitter 111. The flux of light introduced into the polarizing beam splitter 111 is reflected at the most part thereof by the polarizing beam splitter 111 and introduced to the reflection type liquid crystal element 112. Where the white is to be displayed, the flux of light is converted into P polarized light by the reflection type liquid crystal element 112 and is introduced back into the polarizing beam splitter 111. The P polarized light passes as it is through the polarizing beam splitter 111, whereafter the flux of light forms an image on the screen through the projection lens. On the other hand, where the black is to be displayed, the flux of light is reflected from the reflection type liquid crystal element 112 while it remains S polarized light and is introduced back to the polarizing beam splitter 111. Then, the S polarized light is reflected by the polarizing beam splitter 111 and returns to the original light path.
Incidentally, the projection type image display apparatus 110 which uses such a conventional reflection type liquid crystal element as described above has the following problems.
The linearly polarizing element 113 is located in front of the polarizing beam splitter 111 as seen in FIG. 22 so that only a flux of light polarized in one direction, for example, only a flux of S polarized light, is introduced into the polarizing beam splitter 111.
However, a ray of light which is not included in a meridional plane, that is, a skew light ray, includes, when it enters the polarizing beam splitter 111, not only an S polarized light component but also a P polarized light component. If the polarizing beam splitter 111 is ideal, then the P polarized light component passes through the polarizing beam splitter 111 and does not illuminate the reflection type liquid crystal element 112. Actually, however, also the P polarized light is partly reflected by the polarizing beam splitter 111 and enters the liquid crystal element.
As a result, for example, when the black is to be displayed, the P polarized light reflected by the polarizing beam splitter 111 is reflected by the reflection type liquid crystal element 112 and enters the polarizing beam splitter 111 again. Thereupon, most of the P polarized light passes through the polarizing beam splitter 111 and is projected to the screen, resulting in deterioration of the contrast of the image.